drm: Mark constant arrays of drm_display_mode const

... and fixup some methods to accept the constant argument.

Now that constant module arrays are loaded into read-only memory, using
const appropriately has some benefits beyond warning the programmer
about likely mistakes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>